Structural characterization of ellagitannin-rich fractions from leaves of three Sonneratia species, and their antioxidant activity and ?-amylase inhibitory effect and mechanism

The objectives of this study were to investigate the structural characterization, antioxidant activity, and ?-amylase inhibitory effect and mechanism of ellagitannin-rich fractions (ETRF) from the mangrove leaves of three Sonneratia species, namely S. apetala, S. caseolaris, and S. alba. It was found that the ETRF from these Sonneratia species characterized by MALDI-TOF-MS and reversed-phase HPLC consisted a complex mixture of oligomeric ellagitannins. All samples possessed similar mass distributions, but different polymer chain length, with the longest for S. caseolaris up to pentamers and the shortest for S. alba up to trimers. Acid hydrolysis of the ETRF resulted in substantial amounts of gallic acid and ellagic acid, which the concentration of ellagic acid was significantly higher than that of gallic acid. The ETRF displayed excellent inhibition against ?-amylase activity with IC50 values of 10.12 ? 0.08, 7.96 ? 0.05, and 11.96 ? 0.07 ?g/mL for S. apetala, S. caseolaris, and S. alba, respectively. Analysis of fluorescence data revealed that the quenching mechanism of ?-amylase fluorescence by ETRE was the static quenching and the complexes between them might be formed. It was also observed that the ETRF possessed stronger DPPH and ABTS radical scavenging activities, as well as ferric reducing powers than that of synthetic antioxidant BHA. Results of this study are valuable not only for elucidating ellagitannin compositions of three Sonneratia species but also for further exploiting them as a potential source of natural antioxidant agents and ?-amylase inhibitors.